Laser-aided direct metal manufacturing is defined as rapid near-net shaping that can rapidly manufacture 3D products and tools necessary for the manufacture of the products with functional materials (for example, metal, alloy, ceramic or the like) on the basis of the digital data of 3D subjects stored in computers, and falls under “direct metal tooling”.
The digital data of 3D subjects includes 3D Computer Aided Design (CAD) data, medical Computer Tomography (CT) and Magnetic Resonance Imaging (MRI) data, and digital data measured by 3D object digitizing systems, and the tools denote trial and mass-production molds and dies necessary for the manufacture of products.
Those techniques allow functional metal prototypes, trial and mass-production molds, finished products of complicate shape and various tools to be rapidly manufactured in comparison with conventional manufacturing techniques such as Computerized Numerical Control (CNC) cutting, casting, and other manufacturing machinery, etc. Those techniques are applicable to the restoration, remodeling and repairing of molds and dies using reverse engineering technology.
The underlying concept of those techniques, in which physical shapes can be generated from CAD data, is similar to that of general printers. The direct metal manufacturing allows 3D physical shapes to be generated by forming functional material in 3D space at the precise positions, much as printers print documents by applying carbon or ink on the 2D surface of paper at precise positions using document data files stored in computers.
Since it is difficult to generate 3D shapes from CAD data through conventional manufacturing processes in which a material is cut or a molten material is poured into and solidified in a mold, Materials Incress Manufacturing (MIM) has to be employed.
Basically, a 3D object is comprised of 2D surfaces, and each of the 2D surfaces is comprised of 1D lines. Accordingly, a 3D shape can be manufactured by stacking 2D surfaces one on top of another. This technique is called MIM process. As shown in FIG. 1, the 3D shape is manufactured through additive materials deposition for building shapes, differently from the conventional manufacturing processes in which a bulk material is cut or a molten metal is poured to a mold and solidified in the mold.
In the laser-aided direct metal manufacturing technology, the 2D surfaces are physically formed through laser cladding.
As shown in FIG. 2, the laser cladding is a technology of forming a cladding layer 205 on the surface of a specimen in such a way that a local molten pool 203 is formed by irradiating a laser beam 202 on the surface 201 of a specimen and, at the same time, a cladding material (for example, metal, alloy, ceramic or the like) in the form of powder is fed to the molten pool 203 from the outside. Referring to FIG. 3, in the laser-aided direct metal manufacturing, 3D functional metallic products or tools can be rapidly shaped in such a way that 2D sectional information is calculated from 3D CAD data and cladding layers each having shape, thickness and/or height corresponding to the 2D sectional information are sequentially formed.
The 2D sectional information used in the process of shaping is made by slicing 3D CAD data into data of a uniform thickness and/or a uniform height or into 2D data of a variable thickness, which is utilized as shaping information. In order to physically materialize a precise 3D shape corresponding to CAD data by using the sectional data, a cladding layer of precise shape, height and/or thickness corresponding to the 2D sectional information can be formed through the laser cladding process.
The above process considerably affects the dimensional precision of a 3D product. In particular, a technology of controlling the height of a cladding layer is a key technology in implementation of the laser-aided direct metal manufacturing technology.
In the laser-aided direct metal manufacturing technology, as in the laser cladding technology shown in FIG. 2, a cladding layer is formed by interpolation-transferring a metallic substrate (hereinafter, referred to as “a specimen”) around a fixed laser beam along x and y-axes or a laser beam around a fixed specimen. Alternatively, the laser beam can be transferred together with the specimen, and a three or more-axis transfer system or robot can be utilized to increase the degree of freedom of manufacturing.
In the process of shaping, the shape of the cladding layer corresponding to the 2D sectional information mainly depends on a tool path calculated from the sectional information and the precision of a transfer system, and can be relatively easily materialized. However, the height of the laser cladding layer is affected by a large number of process parameters, such as a laser power, the mode and size of a laser beam, the traverse speed of a specimen, the characteristics of cladding powder, powder feeding rate, the falling speed of powder, the overlapping factor of cladding beads, the kinds or amounts of various auxiliary gases supplied, etc. Additionally, environmental factors, such as the variation of temperature on the surface of a specimen caused by heat accumulation, the conditions of the surface of a specimen and a laser generator, can affect the height of the cladding layer formed.
Accordingly, in order to obtain the height of the cladding layer corresponding to 2D sectional information, there is technical difficulty that process parameters affecting the height of the cladding layer should be controlled while the position of a molten pool is monitored in real time.
U.S. Pat. No. 6,122,564 discloses a feedback apparatus and method that is comprised of an optical detection device using a phototransistor and an electron circuit for the purpose of controlling the height of a cladding layer. In this method, the optical detection device is positioned near a molten pool formed on the surface of a specimen by the irradiation of a laser beam and the optical axis of the optical detection device is arranged toward a target height so as to detect light (light of an infrared wavelength) irradiated from the molten pool when the molten pool reaches the target height. The optical detection device is comprised of a narrow band-pass filter, a camera lens, a phototransistor or photoelectron sensor. In order to allow light (infrared light) to be detected by the phototransistor only when the molten pool reaches a height at which the molten pool meets the optical axis, a mask having an aperture whose center passes through the optical axis is positioned in front of the phototransistor.
As a result, when the molten pool reaches a target height (the height of a cladding layer reaches the target value), part of light having only an infrared light wavelength passes through the narrow band-pass filter and can pass through the aperture of the mask, so the phototransistor can detect the light. However, when the molten pool does not reach the target height, light irradiated from the molten pool is blocked by the mask, so the phototransistor cannot detect any light.
In this method, it is determined whether the height of a cladding layer (a molten pool) reaches a target value through the light detection of the phototransistor. The phototransistor experiences a voltage drop phenomenon when being exposed to light. In this case, an electric circuit is constructed such that an analog voltage signal transmitted to a laser generator is controlled using an electric signal generated, and a laser power is controlled by allowing laser beam to be On or Off according to the detection of light by the phototransistor.
However, in U.S. Pat. No. 6,122,564, the optical detection device determines the same if the height of the molten pool is greater or less than a target value of the cladding layer. At this time, there occurs a problem that a normal laser power is generated. In particular, when at a certain position the height of the cladding layer is partially greater than the target value, the optical detection device determines that the height of the cladding layer does not reach the target value, and generates a normal laser power.
Accordingly, the cladding layer at this position is coated to be rather thicker or higher and the repeated performance of the laser cladding at this position causes the problem to be worse, thus deteriorating the precision of shaping. Additionally, in the laser-aided direct metal manufacturing, when a 3D shape is formed using 2D sectional information of a uniform thickness and/or height, there occurs no problem with the mechanical structure of the optical detection device. However, when the 3D shape is formed using 2D sectional information of a variable thickness and/or height, there occurs a problem that the optical detection device should be arranged and corrected whenever the height of the cladding layer is varied.
In addition, a laser power control method is a laser beam On/Off method in which the duration time of a laser pulse is controlled, so it is difficult to apply the technology to a continuous wave laser generator.